This invention relates to an isolation process for a dihydroxydiphenyl sulfone isomer. More particularly, it is concerned with a process for isolating a high purity 4,4'-dihydroxydiphenyl sulfone from an isomer mixture consisting of 4,4-dihydroxydiphenyl sulfone and 2,4'-dihydroxydiphenyl sulfone.
4,4'-Dihydroxydiphenyl sulfone, due to the uniqueness of its diphenyl sulfone linkage, has a heat resistance, a resistance to oxidation and a stability to light, and because of these characteristics 4,4'-dihydroxydiphenyl sulfone has a recent tendency to its increasing utilization as a substitute for bisphenol A in the field of plastics such as polyester resins, epoxy resins and polycarbonate resins.
In general, as a process for preparing 4,4'-dihydroxydiphenyl sulfone there are known a process wherein phenol and a sulfonating agent such as a concentrated sulfuric acid, a fuming sulfuric acid or a sulfuric anhydride are reacted with each other, and a process wherein phenol and p-phenolsulfonic acid are reacted with each other. According to those manufacturing processes, however, not only it is impossible to avoid the formation as a side reaction product of 2,4'-dihydroxydiphenyl sulfone, an isomer of 4,4'-dihydroxyphenyl sulfone, but also it is not easy to isolate 4,4'-dihydroxydiphenyl sulfone from this isomer mixture, therefore, a fairly large amount of 2,4'-isomer is contained in ordinary industrial products.
As previously noted, 4,4'-dihydroxydiphenyl sulfone is a compound which is not having many application aspects in the field of high polymer industry as a substitute for bisphenol A. In this case, a high polymer prepared from 4,4'-dihydroxydiphenyl sulfone containing 2,4'-isomer is small in molecular weight and exhibits deterioration in mechanical properties as compared with one prepared from 4,4'-dihydroxyphenyl sulfone not containing 2,4'-isomer, and the larger the content of 2,4'-isomer the more noticeable becomes this tendency. For effective industrial utilization of 4,4'-dihydroxydiphenyl sulfone, therefore, it is necessary to remove the coexistent 2,4'-dihydroxydiphenyl sulfone and isolate 4,4'-dihydroxydiphenyl sulfone in high purity.
Heretofore, as a purification process for 4,4'-dihydroxydiphenyl sulfone there have been known recrystallization from water, recrystallization from an aqueous methanol solution, or washing with a hot aqueous solution at above 120.degree. C. containing an aliphatic higher alcohol (see Japanese Patent Publication No. 3005/67). However, these processes are effective for removing colored impurity and resinous substance contained in a crude dihydroxydiphenyl sulfone mixture and are not effective for removing the 2,4'-dihydroxydiphenyl sulfone isomer. Therefore, as a process for separating 2,4'-dihydroxydiphenyl sulfone, there have been proposed a separation process based on the formation of a calcium complex (see U.S. Pat. No. 2,392,137) and a separation process based on the formation of a benzene adduct [see "Journal of Chemical Society" (1949), pp. 2854-2856], but these processes are industrially unsuitable because they require complicated operations.
As an industrial process for the separation of 2,4'-dihydroxydiphenyl sulfone there have been proposed a process using sym-tetrachloroethane as an extraction solvent (see Japanese Patent Publication No. 5274/63), a process using o-dichlorobenzene as an extraction solvent (see Japanese Patent Publication No. 24660/68) and a process using mono-, di- and trialkylphenol as extraction solvents (see Japanese Patent Publication No. 43936/72). However, the solubility of 2,4'-dihydroxydiphenyl sulfone in those solvents at ordinary temperature is so low that it is difficult to fully separate 2,4'-dihydroxydiphenyl sulfone from the isomer mixture at ordinary temperature. In view of this point, all of the aforesaid three processes adopt a high temperature treatment (100.degree.-150.degree. C.) such as a hot filtration for increasing the solubility of 2,4'-dihydroxydiphenyl sulfone in those solvents thereby improving the effect of its separation from the isomer mixture. In such a high temperature treatment, however, drawbacks are unavoidable, such as the complexity of operation, damage of filter materials, etc., contamination of the working environment caused by solvent vaporization and the resulting hygienic problems. Thus, those processes are not considered to be fully satisfactory processes industrially.